In typical two-dimensional emission imaging applications, a source of penetrating radiation is administered to the patient. Typically, this consists of a radiopharmaceutical capable of gamma ray emission. So-called "scintillation cameras" are capable of imaging an entire organ, such as the brain, without detector motion. In such cameras, a set of parallel lead collimated holes defines a set of columnar trajectories generally perpendicular to the camera face. The detector is constructed to be position sensitive, i.e., the trajectory from whence a gamma ray originated is identified. This permits direct construction of a two-dimensional projected image by an intensity format on a cathode ray tube and a hard copy such as film.
While two-dimensional radionuclide emission techniques are widely used, it has been recognized that they suffer from significant drawbacks. Thus, a three-dimensional radionuclide distribution in the interior of an object under examination appears with its details from front to back superimposed. Consequently, the resulting two-dimensional image is often difficult to interpret in that concentrations of activity within small volumes are often not identifiable or adequately pinpointed.
Techniques for three-dimensional image reconstruction of the distribution of gamma-emitting radionuclides have also been developed. These techniques, which are usually referred to as "single photo-emission computed tomography" (SPECT), require a large number, e.g. 60 to 120, of two-dimensional images to be recorded from different angles-of-view encircling the source object. When scintillation cameras are used to obtain these images a camera having a planar detector is conventionally employed. It is positioned with its planar crystal surface parallel to an axis of rotation through the source object. Separate images are obtained with the camera rotated sequentially by .GAMMA..theta. until an entire sequence of images (through 360 degrees) encircling the object is obtained. This set of two-dimensional images contains the data necessary for three-dimensional transaxial image reconstruction by conventional means, as disclosed in U.S. Pat. No. 4,095,107.
Typically, such devices have limited sensitivity because the gamma rays are detected only in one direction, one view at a time. The sensitivity may be increased by adding a second camera, but further sensitivity cannot be increased in this manner since two is about the upper limit on the number of cameras because they are bulky and would not readily fit together. The cameras are heavy because of the lead shielding. Their weight is on the order of thousands of pounds. Mechanical rotation of them is therefore difficult and costly. The rotation of these cameras necessarily results in some measure of misalignment, which will degrade the image. Alignment is made more difficult by the great weight and size of the cameras. Further, image non-uniformity errors reinforce upon reconstruction of the three-dimensional images because irregularities and non-linearities in position analysis in each view are in phase with each other since each view has the same error of profile. In addition, due to the motion of the camera, the accuracy of the imaging process is distorted because of the changing orientation of the camera with respect to the environmental magnetic field, such as the earth's magnetic field.